Full blog post: https://withblue.ink/2020/10/03/go-webassembly-http-requests-and-promises.html

Go, WebAssembly, HTTP requests and Promises

# The MIT License

Copyright (c) 2020, Alessandro Segala

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.

http-request-wasm.go

// Copyright (C) 2020 Alessandro Segala (ItalyPaleAle)
// License: MIT

// MyGoFunc fetches an external resource by making a HTTP request from Go
// The JavaScript method accepts one argument, which is the URL to request
func MyGoFunc() js.Func {
	return js.FuncOf(func(this js.Value, args []js.Value) interface{} {
		// Get the URL as argument
		// args[0] is a js.Value, so we need to get a string out of it
		requestUrl := args[0].String()

		// Handler for the Promise
		// We need to return a Promise because HTTP requests are blocking in Go
		handler := js.FuncOf(func(this js.Value, args []js.Value) interface{} {
			resolve := args[0]
			reject := args[1]

			// Run this code asynchronously
			go func() {
				// Make the HTTP request
				res, err := http.DefaultClient.Get(requestUrl)
				if err != nil {
					// Handle errors: reject the Promise if we have an error
					errorConstructor := js.Global().Get("Error")
					errorObject := errorConstructor.New(err.Error())
					reject.Invoke(errorObject)
					return
				}
				defer res.Body.Close()

				// Read the response body
				data, err := ioutil.ReadAll(res.Body)
				if err != nil {
					// Handle errors here too
					errorConstructor := js.Global().Get("Error")
					errorObject := errorConstructor.New(err.Error())
					reject.Invoke(errorObject)
					return
				}

				// "data" is a byte slice, so we need to convert it to a JS Uint8Array object
				arrayConstructor := js.Global().Get("Uint8Array")
				dataJS := arrayConstructor.New(len(data))
				js.CopyBytesToJS(dataJS, data)

				// Create a Response object and pass the data
				responseConstructor := js.Global().Get("Response")
				response := responseConstructor.New(dataJS)

				// Resolve the Promise
				resolve.Invoke(response)
			}()

			// The handler of a Promise doesn't return any value
			return nil
		})

		// Create and return the Promise object
		promiseConstructor := js.Global().Get("Promise")
		return promiseConstructor.New(handler)
	})
}

http-streaming-request-wasm.go

// Copyright (C) 2020 Alessandro Segala (ItalyPaleAle)
// License: MIT

// MyGoFunc fetches an external resource by making a HTTP request from Go
// The JavaScript method accepts one argument, which is the URL to request
func MyGoFunc() js.Func {
	return js.FuncOf(func(this js.Value, args []js.Value) interface{} {
		// Get the URL as argument
		// args[0] is a js.Value, so we need to get a string out of it
		requestUrl := args[0].String()

		// Handler for the Promise
		// We need to return a Promise because HTTP requests are blocking in Go
		handler := js.FuncOf(func(this js.Value, args []js.Value) interface{} {
			resolve := args[0]
			reject := args[1]

			// Run this code asynchronously
			go func() {
				// Make the HTTP request
				res, err := http.DefaultClient.Get(requestUrl)
				if err != nil {
					// Handle errors: reject the Promise if we have an error
					errorConstructor := js.Global().Get("Error")
					errorObject := errorConstructor.New(err.Error())
					reject.Invoke(errorObject)
					return
				}
				// We're not calling res.Body.Close() here, because we are reading it asynchronously

				// Create the "underlyingSource" object for the ReadableStream constructor
				// See: https://developer.mozilla.org/en-US/docs/Web/API/ReadableStream/ReadableStream
				underlyingSource := map[string]interface{}{
					// start method
					"start": js.FuncOf(func(this js.Value, args []js.Value) interface{} {
						// The first and only arg is the controller object
						controller := args[0]

						// Process the stream in yet another background goroutine,
						// because we can't block on a goroutine invoked by JS in Wasm
						// that is dealing with HTTP requests
						go func() {
							// Close the response body at the end of this method
							defer res.Body.Close()

							// Read the entire stream and pass it to JavaScript
							for {
								// Read up to 16KB at a time
								buf := make([]byte, 16384)
								n, err := res.Body.Read(buf)
								if err != nil && err != io.EOF {
									// Tell the controller we have an error
									// We're ignoring "EOF" however, which means the stream was done
									errorConstructor := js.Global().Get("Error")
									errorObject := errorConstructor.New(err.Error())
									controller.Call("error", errorObject)
									return
								}
								if n > 0 {
									// If we read anything, send it to JavaScript using the "enqueue" method on the controller
									// We need to convert it to a Uint8Array first
									arrayConstructor := js.Global().Get("Uint8Array")
									dataJS := arrayConstructor.New(n)
									js.CopyBytesToJS(dataJS, buf[0:n])
									controller.Call("enqueue", dataJS)
								}
								if err == io.EOF {
									// Stream is done, so call the "close" method on the controller
									controller.Call("close")
									return
								}
							}
						}()

						return nil
					}),
					// cancel method
					"cancel": js.FuncOf(func(this js.Value, args []js.Value) interface{} {
						// If the request is canceled, just close the body
						res.Body.Close()

						return nil
					}),
				}

				// Create a ReadableStream object from the underlyingSource object
				readableStreamConstructor := js.Global().Get("ReadableStream")
				readableStream := readableStreamConstructor.New(underlyingSource)

				// Create the init argument for the Response constructor
        			// This allows us to pass a custom status code (and optionally headers and more)
        			// See: https://developer.mozilla.org/en-US/docs/Web/API/Response/Response
				responseInitObj := map[string]interface{}{
					"status":     http.StatusOK,
					"statusText": http.StatusText(http.StatusOK),
				}

				// Create a Response object with the stream inside
				responseConstructor := js.Global().Get("Response")
				response := responseConstructor.New(readableStream, responseInitObj)

				// Resolve the Promise
				resolve.Invoke(response)
			}()

			// The handler of a Promise doesn't return any value
			return nil
		})

    		// Create and return the Promise object
    		// The Promise will resolve with a Response object
		promiseConstructor := js.Global().Get("Promise")
		return promiseConstructor.New(handler)
	})
}

js-dictionary-from-wasm.go

// Copyright (C) 2020 Alessandro Segala (ItalyPaleAle)
// License: MIT

package main

// Import the package to access the Wasm environment
import (
	"syscall/js"
)

// Main function: it sets up our Wasm application
func main() {
	// Define the function "MyGoFunc" in the JavaScript scope
	js.Global().Set("MyGoFunc", MyGoFunc())
	// Prevent the function from returning, which is required in a wasm module
	select {}
}

// MyGoFunc returns a JavaScript function
func MyGoFunc() js.Func {
	return js.FuncOf(func(this js.Value, args []js.Value) interface{} {
		// Return a JS dictionary with two keys (of heterogeneous type)
		return map[string]interface{}{
			"hello":  "world",
			"answer": 42,
		}
	})
}

js-native-object-from-wasm.go

// Copyright (C) 2020 Alessandro Segala (ItalyPaleAle)
// License: MIT

// MyGoFunc returns a Go time.Time to JavaScript
func MyGoFunc() js.Func {
	return js.FuncOf(func(this js.Value, args []js.Value) interface{} {
		// Get the current time as a Go time.Time object
		now := time.Now()
		// Get the Date object constructor from JavaScript
		dateConstructor := js.Global().Get("Date")
		// Return a new JS "Date" object with the time from the Go "now" variable
		// We're passing the UNIX timestamp to the "Date" constructor
		// Because JS uses milliseconds for UNIX timestamp, we need to multiply the timestamp by 1000
		return dateConstructor.New(now.Unix() * 1000)
	})
}

js-promise-from-wasm.go

// Copyright (C) 2020 Alessandro Segala (ItalyPaleAle)
// License: MIT

// MyGoFunc returns a Promise that resolves after 3 seconds with a message
func MyGoFunc() js.Func {
	return js.FuncOf(func(this js.Value, args []js.Value) interface{} {
		// Handler for the Promise: this is a JS function
		// It receives two arguments, which are JS functions themselves: resolve and reject
		handler := js.FuncOf(func(this js.Value, args []js.Value) interface{} {
			resolve := args[0]
			// Commented out because this Promise never fails
			//reject := args[1]

			// Now that we have a way to return the response to JS, spawn a goroutine
			// This way, we don't block the event loop and avoid a deadlock
			go func() {
				// Block the goroutine for 3 seconds
				time.Sleep(3 * time.Second)
				// Resolve the Promise, passing anything back to JavaScript
				// This is done by invoking the "resolve" function passed to the handler
				resolve.Invoke("Trentatré Trentini entrarono a Trento, tutti e trentatré trotterellando")
			}()

			// The handler of a Promise doesn't return any value
			return nil
		})

		// Create and return the Promise object
		promiseConstructor := js.Global().Get("Promise")
		return promiseConstructor.New(handler)
	})
}

js-promise-with-errors-from-wasm.go

// Copyright (C) 2020 Alessandro Segala (ItalyPaleAle)
// License: MIT

// MyGoFunc returns a Promise that fails with an exception about 50% of times
func MyGoFunc() js.Func {
	return js.FuncOf(func(this js.Value, args []js.Value) interface{} {
		// Handler for the Promise
		handler := js.FuncOf(func(this js.Value, args []js.Value) interface{} {
			resolve := args[0]
			reject := args[1]

			// Run this code asynchronously
			go func() {
				// Cause a failure 50% of times
				if rand.Int()%2 == 0 {
					// Invoke the resolve function passing a plain JS object/dictionary
					resolve.Invoke(map[string]interface{}{
						"message": "Hooray, it worked!",
						"error":   nil,
					})
				} else {
					// Assume this were a Go error object
					err := errors.New("Nope, it failed")

					// Create a JS Error object and pass it to the reject function
					// The constructor for Error accepts a string,
					// so we need to get the error message as string from "err"
					errorConstructor := js.Global().Get("Error")
					errorObject := errorConstructor.New(err.Error())
					reject.Invoke(errorObject)
				}
			}()

			// The handler of a Promise doesn't return any value
			return nil
		})

		// Create and return the Promise object
		promiseConstructor := js.Global().Get("Promise")
		return promiseConstructor.New(handler)
	})
}